The present invention relates to a brush. More particularly, the invention relates to a brush for a starter generator, and especially for an aircraft starter generator.
Aircraft turbine engines require a starter much the same as a car, except that the turbine is free wheeling devices. Thus, a motor could act as a starter. To save weight, aircraft manufacturers utilize a DC generator as a starting device by motorizing the generator. Aircraft starter-generators are designed in sizes of 150, 250, 300 and 400 amperes and are designed to carry very small overloads in order to keep their weight to a minimum. In the starting mode, a 300 ampere generator will draw 1700 to 2000 amperes of current, thus overloading the starter-generator by a large amount of current. In one to two seconds, the 1700 to 2000 ampere current draw will reduce to 800 to 1200 amperes. This can last for an additional one to two seconds, tapering down 800 amperes for an additional six to eight seconds. Brush temperatures can exceed 800 to 1000 degrees F in three to four seconds. The duty cycle of the starter-generator in the starter mode is about 30 seconds. The short duty cycle allows the starter to exist without burning up.
The weakest link in the design of a starter-generator is the brushes. The carbon brushes utilized in starter-generators wear rapidly when brush temperatures exceed 450 degrees F. Therefore, the life of the starter-generator is governed by the life of the brushes. For the past 25 years there have been no major brush design movements in aircraft starter-generators, to increase brush life. With the development of higher horsepower turbine engines the 300 to 400 ampere starter-generators utilized to start such engines have been experiencing rapid brush wear. Burning brush boxes and brush heat cause the carbon to turn red and then white, as the heat increases.
Inspection and testing of known brushes have uncovered various deficiencies in various parts of the brush. The present brushes use a single brush shunt. While sufficient in size to carry generating loads, it is deficient in its capability of carrying starting loads. The presently used brush terminal acts as a resistance point to current flow along with the single brush shunt. The brush cannot dissipate the heat created, due to the high resistances in the shunt and terminal, causing annealing of the commutator bars and their subsequent movement, which destroys the brush.
The principal object of the invention is to provide a starter-generator brush having increased strength, reduced operating temperatures and increased current flow with minimal heat.
An object of the invention is to provide a starter-generator brush having great brush strength, sufficient to prevent carbon breakage at speeds greater than 14,000 RPM.
Another object of the invention is to provide a starter-generator brush having a twin shunt and a hammer plate which strengthens the brush sufficiently to prevent carbon breakage at speeds greater than 14,000 RPM.
Still another object of the invention is to provide a starter-generator brush having reduced operating temperatures.
Yet another object of the invention is to provide a starter-generator brush having operating temperatures reduced by 40% relative to single shunt brushes.
Another object of the invention is to provide a starter-generator brush having copper plates which heat sink the carbon wafers and reduce brush operating temperatures.
Still another object of the invention is to provide a starter-generator brush having a hammer plate on one wafer of each pair of carbon wafers which reduces the brush operating temperatures.
Yet another object of the invention is to provide a starter-generator brush having copper plates on the wafers of each pair of carbon wafers which increase the current flow with reduced heat.
Another object of the invention is to provide a starter-generator brush which transmits over 40% more current than known brushes.
Still another object of the invention is to provide a starter-generator brush which is able to transmit up to 2000 amperes with minimal heat.
Yet another object of the invention is to provide a starter-generator brush having a shunt connection which is able to transmit up to 2000 amperes with minimal heat.
Another object of the invention is to provide a starter-generator brush having a terminal which is capable of carrying 2000 amperes with minimal heat.